A magnetic core has the effect of accelerating induction heating by concentrating magnetic force lines on a workpiece and increasing the power of a coil in the case where the magnetic core is mounted on a rear surface of the coil, and has the effect of preventing a part requiring no hardening operation from being heated by shielding the part against magnetic force lines in the case where the magnetic core is mounted on a front surface of the coil. Thus, a magnetic core is a component part indispensable for a heating coil of an induction hardening apparatus.
For example, if a workpiece to be subjected to induction hardening has a complicated shape which necessitates adjustment of hardening depth, it is possible to change the state of induction heating and control the hardening depth of the workpiece by altering the shape, size, quantity, direction, and position of the core to be mounted on a heating coil. Materials of such cores should have some magnetic properties such as (1) satisfactory frequency characteristics, namely, small changes of inductance at varied frequencies, (2) a high saturation magnetic flux density, (3) a high relative permeability, and (4) a low iron loss.
Because target workpieces have various shapes, most core parts are produced in a high-mix low-volume production manner, including one-by-one cutting operations in many cases. Accordingly, there is a need for core materials having high strength and good cutting workability.
Magnetic cores manufactured by using powder-metallurgy techniques, which can reduce raw material losses and have a high mass-production capacity, are commonly used as magnetic cores for heating coils of induction hardening apparatuses. Examples of magnetic cores for induction hardening coils include a product made of magnetic powder particles consolidated with a fluororesin, and a product made of sendust particles consolidated with a phenol resin. However, these magnetic cores have been disadvantageous in that they have relatively poor material strength, and may be cracked when their thin sections are subjected to cutting operations, or broken upon mounting on coils.
In an attempt to solve the above-mentioned problems, it has been proposed by the present applicant to produce a magnetic core by compression molding and thereafter thermally hardening an iron-based soft magnetic powder having resin films formed on surfaces of particles thereof (Patent Document 1). This magnetic core is one which exhibits high economic efficiency, good magnetic properties and high material strength as well as abroad utility which allows the core to be mounted on a heating coil portion of an induction hardening apparatus.